In the natural state, the genus equidae, horses, mules, donkeys, etc., are grazing animals. Unlike a ruminant, the gut capacity of these animals in relation to their body size is small. This requires the animal to consume small amounts of food at relatively frequent intervals.
The diet of the horse is largely composed of materials high in cellulose, such as hay or pasture grass. The digestive enzymes produced in the glandular stomach and small intestine are not effective in the degradation of these materials. The horse is largely dependent upon the bacteria resident in its caecum and colon to fermentatively break down the cellulose into substances which can be then absorbed and utilized directly.
The efficacy of fermentation in the horse is dependent upon substrate availability. In a natural state, horses and similar animals, because of the continuous ingestion of small amounts of feed, specifically forage, have a relatively constant flow of substrate in the form of ingesta, available for hind gut bacterial, fermentative use. However, under current husbandry practices, this is often not the case. Instead, animals consume a significant portion of their diet in the form of small grains, ingested over a short period of time. This results in erratic movement of ingesta through the GI tract. This material often reaches the large bowel relatively undegraded. This allows for a bacterial fermentation action that causes significant changes in large bowel pH, which in turn, results in fluctuations in species and densities of colonic bacterial populations. Nutrient use efficiency is depressed, thus negatively impacting an important part of the digestive process in these type animals. In addition, destabilization of the large bowel has been associated with certain serious metabolic diseases in the horse, such as colic and the formation of enterolythes, both life threatening situations in the horse. Currently there is no known way to control intestinal pH, or stimulate or modify large bowel bacterial populations in the horse, other than feeding methodology and restrictive use of certain ingredients. Considering that modern husbandy practices make his almost impossible to achieve, methods of controlling these key parameters are much needed.